Keywords:
• elementary particles theory;
• time asymmetric quantum theory;
• Z-boson mass and width;
• unified theory of resonance and decay phenomena;
• causality;
• time asymmetry;
• Breit-Wigner scattering amplitude;
• definition of the mass and width of a relativistic resonance
Summary
A unified theory of resonance and decay phenomena is presented for the non-relativistic and the relativistic cases. It incorporates causality and time asymmetry.

The relativistic Quasistable particles are described by semigroup representations of the causal Poincaré transformations characterized by spin j and complex square mass SR = (MR − iΓR/2)2, where ΓR is the width of a relativistic Breit-Wigner scattering amplitude.

The lifetime of the exponentially decaying state described by this semigroup representation [j; SR] is derived as τ = h̄/ΓR. This provides a criterion for the appropriate definition of the mass and width of a relativistic resonance.